Zeolite L has been known for some time as an adsorbent and in U.S. Pat. No. 3,216,789 is described as an alumino-silicate of the formula: EQU 0.9-1.3M.sub.2/n O:Al.sub.2 O.sub.3:5.2-6.9 SiO.sub.2 :yH.sub.2 O
(where M is an exchangeable cation of valency n and y is from 0 to 9) having a characteristic X-ray diffraction pattern.
EP-A-96479 describes a zeolite L which is particularly useful as a catalyst base in hydrocarbon conversions such as aromatization. The zeolite comprises crystallites in the form of cylinders with a mean diameter of at least 0.1 micron, preferably at least 0.5 micron and with an aspect ratio (ratio of cylinder length to diameter) of at least 0.5. The gel from which the zeolite is obtained comprises the following ratios of components: 2.4 to 3.0 moles K.sub.2 O, 0.6 to 1.3 moles Al.sub.2 O.sub.3, 8 to 12 moles of SiO.sub.2 and 120 to 240 moles H.sub.2 O. A particularly preferred gel has the following compositions: EQU 2.62K.sub.2 O:Al.sub.2 O.sub.3: 10SiO.sub.2: 160H.sub.2 O.
The potassium form of zeolite L, hereinafter identified as zeolite KL, may also contain caesium, as described in EP-A-323892.
Typically the zeolite is loaded with one or more metals such as platinum, tin, germanium, rhenium or iridium, particularly platinum, to prepare the desired catalyst.
New forms of zeolite KL are sought which are particularly useful as a catalyst base for aromatization and which permit regeneration of spent catalyst. Imperfections in the zeolite crystals and relatively long uni-directional zeolite channels result in poor utilization of Pt, poor maintenance of catalyst activity, and undesirable secondary reactions. To improve the properties of such a zeolite the channel length of the zeolite should be decreased to well below one micron, but at the same time the surface area of the zeolite crystal should be maintained as large as is practicable, and the crystals should be well-formed i.e. without a significant level of crystal imperfections.
These features are present if well-formed zeolite crystals can be made in the form of very flat cylinders. The present invention provides a zeolite whose crystals have the requisite properties; the present invention also provides a process for producing such zeolites.